This invention relates to the field of multilayer high density interconnect devices, sometimes referred to as multichip modules (MCM). More particularly, this invention relates to the manufacture of substrates for MCM's, and the resulting MCM substrate product, in a process using a thermoplastic fluoropolymer composite material, additive copper plating, insulation lamination, planarization, sputter deposition of copper seed layers and where required, formation of a rigid top layer.
The need for and desireability of substrates for MCM's is well known in the electronic industry. The need for increased density in I/C packaging, increased interconnection capacity, and improved interconnection performance is well known among those involved in VLSI packaging and elsewhere in the electronics industry.
A multichip module can be viewed as a packaging technique in which several I/C chips, which may include complex microprocessor chips, memory chips, etc., are interconnected by a high density substrate. Although substrates for MCM's have been known in general for several years, these known prior art substrates for MCM's typically use thin film polyimide based material systems, and those systems have known deficiencies. Polyimide dielectric materials suffer from poor thermo-mechanical reliability and stability and electrical performance limitations. The polyimide materials are thermosetting, and have high elastic modulus, and there is a significant mismatch, i.e., difference, between the coefficient of thermal expansion (CTE) of the polyimide and the copper-conductors. That all results in high stresses in the polyimide material and in surrounding materials and interfaces during thermal excursions in either or both the manufacturing process for or during use of the MCM substrate. Those stresses can lead to dimensional instabilities, cracking, delamination and other thermo-mechanically related problems. Also, the chemical bond between the polyimide and the copper conductors is weak and sensitive to the presence of water, thus leading to poor reliability of the copper-polyimide interface.
In some manufacturing processes for polyimide-based substrates for MCM's, a liquid form polyimide precursor is applied and then cured; in other processes, a sheet form polyimide prepreg is used. In either case, it is difficult to keep water out of the MCM substrate structure made with polyimides. Water is generated during the polyimide cure, and the polyimide equilibrium water absorption is substantial, often in excess of 1%, and the rate of water uptake can be high. Also, water diffusion rates in polyimides are often high, so any water in a polyimide MCM substrate structure can diffuse quickly to the polyimide-conductor interface to corrode or otherwise degrade the interface. To combat this interface problem, a layer of metal, usually chromium is used as a barrier layer between the polyimide and the copper conductors. While this use of a barrier layer can be successful, it adds significant and expensive processing steps and costs to the manufacture of the MCM substrate structure.
Another point to note about polyimide based substrates for MCM's is that they are thin film structures. The layers of polyimide are typically in the range of 5 to 12 microns in thickness, with conductor line thickness in the range of 2 to 7 microns. Those relatively thin conductor lines mean relatively high resistance and relatively high loss; and both the nature of the polyimide material, per se, and the relatively thin layers used, result in poor electrical insulation characteristics.